A parallel plate capacitor having plates of area $S$ and plate separation $d$ has capacitance $C_1$ in air. When two dielectrics of different relative permittivities $\left({\epsilon }_1=2\right.$ and $\left.{\epsilon }_2=4\right)$ are introduced between the two plates as shown in the figure, the capacitance becomes $C_2.$ The ratio $\frac{C_2^{\mathrm{y}}}{C_1}$ is,